1. Field of the Invention
The present invention relates to wireless local area network (WLAN) devices and particularly to techniques in WLAN devices for performing startup scans for radar, identifying backup channels for a channel switch, and efficiently changing channels in the event of radar detection in the operating channel.
2. Description of the Related Art
Wireless local area network (WLAN) devices operating in the 5 GHz spectrum coexist with radar systems. Various regulatory standards (e.g. the draft European Telecommunications Standards Institute (ETSI) EN 301 893, version V1.2.1, published 2002-07) require that a dynamic frequency selection (DFS) feature be incorporated in 5 GHz WLAN devices. This DFS feature should switch operating channels in the presence of co-channel radar and uniformly spread operation across a wide frequency range.
Regulatory standards typically provide simple guidelines for radar detection and/or avoidance of co-channel radar. For example, current European regulatory standards require that a 60 second scan of each allowed channel be completed before operation commences. Moreover, upon detection of a radar signal on the current operating channel, the WLAN device (i.e. an access point or a station) must cease communication within a short timeframe, e.g. 0.2-1.0 sec. Note that pending revisions to the European regulatory standards may require that aggregate transmission time of an access point or station be limited to a total of 260 ms counting from the instant a radar is detected. Other regulatory domains have proposed similar guidelines. For example, a draft IEEE 802.11h specification (which adapts the 802.11a standard to European regulations) suggests that stations respond to a channel switch announcement frame from an access point by ceasing data transmission, thereby avoiding harmful interference to the co-channel radar system.
However, these regulatory standards do not provide specific implementation details. Therefore, various proprietary solutions have been proposed. For example, U.S. patent application Ser. No. 10/039,451 (hereinafter McFarland), entitled “Radar Detection And Dynamic Frequency Selection For Wireless Local Area Networks”, filed on Dec. 6, 2001 by Atheros Communications, Inc., and incorporated by reference herein, teaches how to efficiently detect co-channel radar. In McFarland, signal pulses are received as detected events. Any detected events that correspond to network traffic can be eliminated. At this point, any non-eliminated events can be examined to determine whether they correspond to radar signals. This examination can include identifying pulse repetition frequency, a pulse period, or a number of pulses in a pre-defined time period.
McFarland also teaches one technique to cease transmissions between an access point and its associated stations upon detection of radar. In this technique, a point coordination function (PCF), which is provided in the IEEE 802.11 standard, can be used to control which stations transmit and when they transmit. Specifically, a PCF beacon, which is sent by an access point, announces the beginning of a polling period, wherein all stations must wait until polled by that access point before transmitting. During the gap between the PCF beacon and the polling of the next station, the access point can perform a radar detection cycle. At the end of the radar detection cycle, the access point can proceed with normal PCF polling.
Other implementation techniques related to DFS are currently being developed. These techniques should include defining the specific algorithms for performing startup scans for radar, determining an acceptable backup channel for a possible channel switch, and efficiently switching channels in the case of radar detection in the operating channel. Preferably, these techniques should further include a mechanism whereby legacy stations, i.e. those stations that have not implemented the DFS functionality, can be easily controlled by the access point during a channel switching operation and, more often than not, can operate on the new channel.